Direction finder



Au 29, 1950 w. VAN a ROBERTS 2,520,693

DIRECTION FINDER Filed April 22 1948 Patented Aug. 29, 1950 maao'nonmoan Walter van B. Roberts, Princeton, N. J., assi'gnor to RadioCorporation of America, a corporation of Delaware Application April 22,1948, Serial No. 22,599

3 Claims. (CL 343- 113) This invention relates to improvements in radiodirection finders, and more particularly to direction finders for usewith signals subject to multipath propagation or night effect."

It is well known that radio waves of certain lengths travel over two ormore paths under certain conditions of the ionosphere, and that suchwaves, especially when arriving at steep elevation angles, introduceso-called polarization errors in the indications of prior art types ofdirection finders; The effect of polarization error is to produceincorrect and frequently unsteady bearing indications, which sometimeshave no apparent relationship to the actual direction of arrival of thewaves.

The principal object of the present invention is to provide methods andmeans for obtaining substantially correct bearing indications undersevere night efiect conditions.

More specifically, it is an object of this invention to provide methodsand means for sampling the apparent bearings of a radio signal atvarious elevation angles, and selectin therefrom the one which is mostprobably correct.

Another object of the invention is to provide methods and means forevaluating the probable accuracy of bearing indications in general undercurrently prevailing conditions.

The invention will be described with reference to the accompanyingdrawing, wherein:

Figure l is a schematic dia ram of a presently preferred embodiment ofthe invention, and

Figure 2 shows a typical indication provided by the system of Figure l.

The direction finder shown in Figure 1 includes a loop I mounted\intrunnions on a fork 3 for rotation about a horizontal axis. The fork 3is supported on a vertical shaft 5 which extends into, and is supportedby, a bearing 1 on a vertical shaft 9. The fork 3 is rotatable about thevertical axis of the shafts 5 and 9. The lower end of the shaft 9extends into a cup bearing H on a base plate l3.

A motor I5 is secured to the trunnion member 3, and one end of its rotorshaft I1 is provided with a crank IS. A connecting rod 2| extends fromthe end of the crank I9 to a crank 23 which is secured to the shaft 9.The length of the crank 23 is considerably greater than that of thecrank it.

The shaft of the motor I 5 is also provided with a worm 25 engaging aworm wheel 21. The worm wheel 21 is connected to a crank 29, coupledthrough a connecting rod. 3| to a longer crank 2 it. The crank it issecured to the horizontal shaft 85 supporting the loop I.

A worm wheel 31 is secured to the vertical supporting shaft 9, inengagement with a worm 38 on a shaft 4|. The shaft ll is supported in abearing member 43 secured to the base plate It.

A manually operable crank 45 is fastened to the shaft 4|. The worm wheel31 carries a scale disk 41 calibrated in degrees. and a pointer 49 forcooperation therewith is supported on the base plate IS.

The terminals of the loop I are connected to a radio receiver 5| whoseoutput is supplied to the beam intensity control electrode 53 of acathode ray oscilloscope tube 55, The electrode 58 may be normallybiassed, as by a source 51, so that the beam intensity is not sufiicientto produce a Y visible trace on the screen of the tube 55.

A variable voltage divider 58 is produced with its control shaft coupledto the horizontal loop shaft 35, as indicated schematically by the dashline 6|. The body of the voltage divider 59 may be supported on the forkmember 3, so that the position of the movable contact arm 63 on theresistance element depends upon the angular position of the loop I aboutits horizontal axis of rotation. a

A second voltage divider 85 has its control shaft coupled to thevertical loop shaft 5, as indicated by the dash line 81. The body of thevoltage divider B5 is supported on or coupled to the shaft 8, so thatthe position of the movable contact arm 89 on the resistance elementdepends upon the relative angular displacement between the loop shaft 5and the supporting shaft 9.

The voltage dividers 59 and B5 are both connectedacross a D.-C. sourceH, and their movable contact arms 63 and 69 respectively are connectedto the horizontal and vertical deflection elements of the cathode raytube 55.

In the operation of the described direction finder, the motor It runscontinuously to rock the fork 3, and with it the loop I, back and forthin azimuth through an angle of, for example :30 from its mean azimuth atarate of perhaps 7 5 oscillations per second. At the same time, the

loop is swung in elevation, from zero, with its null axis horizontal, tosay Owing to the worm and worm wheel arrangement 25, 21, the motion inelevation is much'slowerthanthat in azimuth, say once every ten seconds.The foregoing rates are given only by way of example,

and either may be varied considerably. .Thus

every possible combination of azimuth and elevation (withinapproximately one degree) included 3 in the total solid angle swept bythe null axis of the loop will be covered every ten seconds.

The voltage at the movable arm 63 of the voltage divider 59 at anyinstant will be proportional to the elevation )118'18 of the loop axis.Similarly, the voltage at the arm 69 of the voltage divider 55 will bedetermined by the azimuth of the loop axis, referred to the angularposition of the shaft 1 and the scale 41. Since these voltages areapplied to the vertical and horizontal deflection elements respectivelyof the tube 55, the cathode ray beam will be deflected to strike thescreen at a point which corresponds to the instantaneous position of theantenna.

The cathode ray beam scansthe face of the tube 55 in a more or lesssquare pattern of horizontal lines, displaced vertically, like atelevision raster. The center of each line corresponds to the meanposition in azimuth of the loop I. Since the beam is normally biassedoff by the source 51, no visible trace appears on the screen of the tube55 except when there is more than a certain minimum output from thereceiver The receiver is tuned to the frequency of operation of thetransmitter whose direction is to be determined. Wave energy from saidtransmitter may arrive at the loop I by three paths: (1) more or lessparallel to the earth's surface, as a ground wave, (2) by way ofreflection from the ionosphere, as a sky wave, and (3) by reflectionfrom the ground, or a combination of reflections from the ionosphere andthe earth.

In general, the voltage induced in the loop will not go to a minimum, inthe presence of two or more of these types of wave propagation, when thenull axis of the loop is horizontal and pointed toward the transmitter.It may or may not be a minimum when the loop axis is pointed in someother horizontal direction.

The operation of the present invention is based on the assumption, whichis ordinarily true, that the various "rays of energy arriving at theloop have one characteristic in common, in that they all lie in thevertical plane including the transmitter and the direction finder. Theresultant energy flow at the loop is therefore in this plane. Hence ifthe loop axis is aligned in elevation as well as in azimuth with theresultant ray at any moment, the resultant magnetic field of this raywill lie in the plane of the loop, whatever the polarization (plane orelliptical) of the field, and no voltage will appear across the loopterminals.

Transmission conditions usuall vary too rapidly to permit directionfinding by manual adjustment of both elevation and azimuth of the loopaxis. Therefore, it is preferable, as in the system of Figure 1, tocause the antenna to scan cyclicall and rapidly through a large numberor directions including that of the resultant ray- Since the loop willrespond to waves arriving from substantially every direction except thatof the null axis, the receiver 5| will provide output during the most ofeach scanning period.

The cathode my will trace luminous lines on the screen of the tube 55,the lines being interrupted at points corresponding to the passage ofthe loop through a position wherein a null or minimum of signal ispicked up,

See Figure 2, which shows a typical display on the face of the cathoderay tube 55. At some elevation angle there will be a null at the correctazimuth. In successive scans, this elevation may be different, with theresult that several null indications will appear at the same azimuth, asillustrated. Other nulls may also be indicated at shown at 13, I5, H and19, will vary in azimuth with variation in transmission conditions, andare thereby distinguishable from the true nulls, which will vary inelevation but remain at the same azimuth.

To provide a numerical indication of the bearing of the transmitter, thewhole oscillating antenna assembly is rotated by means of the crank 45to align the true null indications with a reference mark such as anarrowhead Ii on the screen of the tube 55, as shown in Figure 2. Themean azimuth of the oscillating loop then coincides with that of thearriving waves, and is indicated by the scale 41 against the pointer 49.

Although a specific embodiment of the invention has been described,using an oscillating loop antenna and a cathode ray indicator, it willbe apparent without detailed explanation that other known types ofdirective antennas or antenna systems may be used, and other knownmethods of comparing successively presented azimuth indications may beemployed.

The invention has been described as an improved direction finder fordistinguishing true indications from false indications caused by nighteffect or polarization error. A directive antenna is moved in elevation,as well as in azimuth, and the azimuth indications at various elevationangles are compared to select the most probably correct azimuth.

I claim as my invention:

1. In a radio direction finder system including wave collector meanssubject to polarization error and having a'null axis in the center of asolid angle of minimum responsiveness the method of determining the trueazimuth of arrival of radio waves, which are transmitted from a remotestation, substantially independently of said polarization error,comprising the steps of cyclically varying the direction of said nullaxis both in azimuth and in elevation to periodically orient said axisin substantially each direction within a, prescribed second solid anglewhich is many times larger than said first-mentioned solid angle andincludes a line to said remote station, identifying, according to boththe azimuth and the elevation of said axis, each of a plurality ofminimum responses to said waves which minimum responses occurrespectively for different directions of elevation of said axis and mayinclude some whose occurrences, because of polarization error effects,are not for the true direction in azimuth of said axis, and selecting asthe true azimuth that azimuth on which, at different elevations, apreponderance of said signals is received.

2. In a radio direction finder adapted for use at frequencies exhibitingmultipath transmission and night effects for determining the trueazimuth of arrival of radio waves transmitted from a remote station, aloop antenna having a null axis in the center of a solid angle ofminimum responsiveness, said loop being subject to polarization errors,means for cyclically rotating said loop both in azimuth and in elevationto periodically orient said axis in substantially each direction withina prescribed second solid angle which is many times larger than saidfirst-mentioned solid angle and includes a line to said remote station;a radio receiver connected to said antenna, and indicator meansconnected to said receiver, said indicator means including a cathode rayoscilloscope tube, means deflecting the oathode ray beam of said tubehorizontally and vertically respectively in synchronism with the motionof said loop in azimuth and elevation, and means responsive to theoutput of said receiver to correspondingly vary the intensity of saidcathode ray beam.

3. In a radio direction finder adapted for use at frequencies exhibitingmultipath transmission and night effects for determining the trueazimuth of arrival of radio waves transmitted from a remote station, aloop antenna, said antenna being subject to polarization errors, meansfor cyclically varying the direction of the null axis of said loop bothin azimuth and in elevation to periodically orient said axis insubstantially each direction within a prescribed solid angle which ismany times larger than a solid angle of minimum responsiveness of saidloop antenna and includes a line to said remote station; a radioreceiver connected to said antenna, and indicator means connected tosaid receiver, said indicator means including means displaying nullswhich occur in the response of said receiver for different directions ofelevation of said axis and which may include some whose occurrences,because of polarization error efiects are not for the true direction inazimuth of said axis, the nulls being displayed as visible markspositioned with respect to a reference frame having azimuth andelevation coordinates in accordance with the corresponding positions ofsaid directive axis within said first-mentioned solid angle, whereby thetrue azimuth of a signal source is indicated substantially independentlyof said polarization errors by a plurality of said marks lying in asingle line representing said azimuth.

WALTER van B. ROBERTS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,151,549 Becker Mar. 21, 19392,231,929 Lyman Feb. 18, 1941 2,304,446 Eaton Dec. 8, 1942 2,410,666Leek Nov. 5, 1946

